I've got a 4l bottle, never topped it up!! Bought it with about 170 bar in it & its really low now, I'm considering selling it as I always use the pump, I've never bothered to find a shop that fills them.
I'm thinking of buying a buddy bottle day tripper for use with my huntsman, as the huntsman has such a small cylinder & only pumps up to 190bar a day tripper would fill it 4x I recon and then I can pump it up at home when I have the time & (dry) space to do it at my leisure.
Plus itl be easier to cart around... Could even get two & a pump.
I suppose the same idea would work with a 15l bottle in your garage & a day tripper or a 1l bottle....

Hope this helps
Rhys